Presently, with the exception of certain kidney diseases, a causitive therapy vital for the treatment of chronic nephritis has not yet been realized. Most medical treatments currently applied to nephritis therefore aim at relaxation of symptoms and retardation or prevention of progression of the disease. A drug therapy, as well as confinement of daily behavior and dietetics, is the major treatment of chronic nephritis. As a drug therapy, an oral steroid drug is used as the primarily chosen medicine, in addition to conventional anti-platelet drugs. A cocktail treatment, in which four types of drugs (i.e. an immunosuppressive drug, a blood coagulation inhibitor and steroid, an immunosuppressive drug, an anti-platelet drug and blood coagulation inhibitor) simultaneously administered, is applied to the treatment of refractory nephritis and rapidly progressive glomerulo nephritis which are difficult to be cured by conventional drugs (Kidney and Dialysis, 34, 555 (1994)).
On the other hand, efforts for establishing new drugs and methods of treatment for nephritis have been undertaken in recent years. These efforts have matured into the development of an angiotensin converting enzyme inhibitor for the treatment of chronic nephritis or nephrotic syndrome (Nephrol. Dial. Transplant., 6, 936 (1993)) and the development of a novel immunosuppressive drug called cyclosporin which is used for the treatment of refractory nephrotic syndrome (Brit. Med. J., 295, 1165 (1987)), for example. Other drugs in which the introduction of is under study include, a thromboxanes synthetic enzyme inhibitor (Clin. Neph., 30, 276 (1988)), a platelet activating factor antagonist medicine (Kidney Int., 31, 1248 (1987)), various growth stimulating factors such as transforming growth factory-.beta., and cytokines as well as their receptor inhibitors (Nature, 346, 371 (1990)). In addition, LDL apheresis has been applied to the treatment of focal sclerosing glomerulonephritis which presents concomitant with hyper lipemia, and found to be effective for the improvement of proteinuria and retardation of kidney function disorder. (Kidney and Dialysis, 34, 555 (1994)).
Although these various methods of treatment have been proven to exhibit certain clinical effects, a medicine effective for the treatment of nephritis without any adverse side effects has yet to be discovered. Establishment of an effective treatment method for nephritis is essential not only to retard introduction of dialysis to a nephritis patient and improve the quality of life for the patient, but also to improve treatment economy. For these reasons, the development of a medicine which can be effectively applied to the treatment of nephritis has been pursued. The present inventors have also proposed and filed an application for a patent relating to the use of purine derivatives exhibiting an anti-inflammation function as an agent for the treatment of nephritis (Japanese Patent Application Laid-open No. 316158/1995). However, development of a novel and more effective drug for the treatment of nephritis has long been anticipated.